If you think that being in a electric field suppose no energy requirement to fuse 2 proton, you are wrong. Actually it is the inverse since the charge interaction is between each other depending of the distance. The energy requirement still is lower for 2 proton to fuse than the predicted model of electromagnetism. That model is not useful anymore when we are talking of very close atomic interaction, you need to good look at quantum mechanic.
Still if we do the calculus based on the electric field and the nuclear force
From the article on wikipedia on the nuclear force it state that the nuclear force overcome the coulomb barrier at around 1.7fm of distance so if we use that as a basis for the distance both proton must be for fusion to occur we have :
U= kQQ/r
http://en.wikipedia.org/wiki/Coulomb_barrier
This give us a energy value when we calculate of:
U= 8.9876e9(N m^2 C^−2)*1.602e-19(C)*1.602e-19(C)/1.7e-15(m) = 135.6e-15(J)
if you convert to eV you get : 846,850eV
A little bit closer than your evaluation. Still it's not close to Gamov energy but is calculus is based on quantum mechanic where you have a quantum tunneling happening.
P.S. don't consider a proton as a capacitor, to have a capacitor you need 2 surface
What if there were no electrons?
Well, for starters, whatever is maintaining the box and plates at that potential with respect to ground is going to have a current sent through it during the operation of adding the dielectric shell, until the entire conducting box-and-plates assembly has no net charge...chrismb wrote:OK, I'll fix that by placing the whole metal box and internal plates in a dielectric box with the inner and outer faces holding a potential gradient and where the inner face has the same potential as the positive charge on the box. That way there will be no preference for the 'holes of electrons' to go anywhere throughout the metal structure.
Bigdealsowhat?
I'm afraid you're talking nonsense. Capacitance works with a potential DIFFERENCE.charge and potential are proportional: charge = capacitance * potential. Providing the capacitance doesn't change so the potential won't change.
Perhaps it will help if you try to think of what charge DENSITY a chunk of matter needs to have if it is at a certain potential. You should quickly realize that there isn't an answer; it depends on the surroundings.
Go read my direct-conversion example again. Make sure you understand it. The concept of the potential function (and its derivative, the electric field) is extremely useful, provided you do realize that the reference level for potential is arbitrary (it's the "+ c" that needs to be added when you integrate the electric field, which IS a measurable physical quantity).
Of course not. The act of adding charge to the box has increased the potential of the box (or the act of applying a potential to the box has caused it to charge up, whichever you prefer), but it has also elevated the potential of the deuterons inside. This is because it's slightly harder to charge up the box if the ions are present, because they repel the 'holes'.But that is a relevant point if you start talking about point charges in a metal box, because if you charged up some 'theoretical' box that only contains two deuterons, would they fuse without any Coulomb repulsion?
The charge on the ions can't change, so when the environment changes, their potential changes instead. You have added energy to them; if they were to leave the box they would accelerate away rapidly on their own, which was not true when the box was grounded.
In fact, a grounded box with a net positive charge inside (from ions, say) will have a net negative charge, because the ions attract electrons from the ground...
Do you believe in electrons?You can see how it begins to get awkward talking about configuration and energy, but we still don't have to resort to declaring a belief in the existence of fields and forces.
I'm getting the impression that you're operating on a freshman-level electrostatics education, which you haven't fully understood. I'm having trouble following your argument, and I'm pretty sure it's not my fault...
No, I don't. Where did you get that from? The argument is that there is no electric field between the protons. See the further detail, below.If you think that being in a electric field suppose no energy requirement to fuse 2 proton, you are wrong.
True, if you talk about a 'capacitor'. I never did talk about a 'capacitor'.P.S. don't consider a proton as a capacitor, to have a capacitor you need 2 surface
But an isolated object has a capacitance with respect to the universe around it, and this can be mathematically shown to be 4.pi.e.a for a spherical surface. Do you want to see the proof?
You keep coming back to the 'net charge' notion. I am talking about an isolated metallic sphere that has a dearth of electrons, such that there is NOT a net charge on it. I do not see why you keep insisting everything must have a net charge? You can remove electrons from an isolated structure and keep all of that within a secondary container that isolates it from fields running to any other potential. Period. It then does not have a zero net charge.Well, for starters, whatever is maintaining the box and plates at that potential with respect to ground is going to have a current sent through it during the operation of adding the dielectric shell, until the entire conducting box-and-plates assembly has no net charge...
Quote:
charge and potential are proportional: charge = capacitance * potential. Providing the capacitance doesn't change so the potential won't change.
I'm afraid you're talking nonsense. Capacitance works with a potential DIFFERENCE.
It will then have an undefined 'potential' which might be mathematically defined as the potential difference with an object that does have a net charge.
Isolated objects don't have to have a zero net charge!
OK. So let's set up the mind experiment again. It is reasonable to probe such a mind-system as one can build in intrinsic errors, but I hope you will eventually see there's nothing significant. here we go; you create a spherical capacitor such that a dielectric spherical shell of some thickness has metallic surfaces inside and out. You ground the outer surface and charge the inner to 1.6MV. Also within this device you put the thin walled metallic sphere I was talking about above, and connect this with a wire of negligible thickness, thus negligible capacitance. The system is held like that for as long as it takes the charge [electrons] on the interior structures to bleed through the dielectric such that at some point the charge on the interior sphere is evenly distributed. Then the thin wire is cut. You should find that the interior thin sphere now lacks some quantity of electrons such that there are more positive charges in it than negative charges, but also the interior lining of the 'capacitor' does not cause the charge that is left in that sphere from becoming non-uniformly distributed.
I was hoping to avoid such a long description originally, but it's fair to question the mind experiment.
So now we have a significantly positively charged sphere. I am now going to try to argue with the field and forces argument: Two protons stuck inside this thing now may experience electrostatic forces. Those two protons may have some sort of potential difference with the sphere and thus are repelled by it, both being postively charged. The two protons, if symmetrically positioned, as you would expect charge to evenly distribute itself, are at the same potential with respect to the sphere. They are symmetrical and identical - why would they be at different potentials??
So if the protons are at the same potential with respect to the sphere, there would be no electrostatic field between them. Electrostatics says that there is a field between dissimilar potentials, but the protons are at the same potential!! So there will be no electrostatic repulsion. By the same electrostatic argument, there should be some potential that can be created on the sphere, by removing its electrons in the manner described above, that actually preferentially pushes the two protons together!
...and freshman electrostatics is wrong?
I'm getting the impression that you're operating on a freshman-level electrostatics education
The problem is that every time a critique of a part of my argument is made, that critique RELIES on a description of the proposition of fields and forces. If you begin with proposition A, then use that to consider proposition B, you will ALWAYS come back to being able to show proposition A is correct in preference to B!! This is an issue of scientific paradigm and using existing orthodoxy to argue paradigm shifts is fraught with all sorts of self-proving, vacuous arguments.
The fields-and-forces arguments being presented against my proposition of energy and configuration are ones that appear to go on to prove themselves, and if so then they prove nothing. What is odd is that I have not made any contention that fields-and-forces is wrong. I've said that there must be a way to contrive the right answer because these are useful and legitimate second-order interpretations. But I've argued that you don't really have to use these second-order observations and that you can actually stick with just the first-order, observed 'facts' if you so chose. Really, that's all that I have tried to argue for.
So there should be no need to go back to fields-and-forces in this debate, you merely have to critisise the logic of energy-and-configuration. Or you could just say 'you can look at it any way you like, I can't see the benefit'. Either is fine, and I rather thought the debate would end up with 'So what!' but we're talking about a fundamental paradigm here which is not easy to see if you're sitting comfortably, right in the middle of that paradigm!
Of course they would! You are generating an electric field between those balls and the ground potentials that surround them. The balls will be pulled in the direction of the e-fields that are 'flowing' down to the ground potentials.
There would be no e-field between the balls (they are at the same potential and you don't get e-fields between objects at the same potential) but there are some on the side facing ground potentials. Unbalanced forces on the balls--->motion of the balls.
There would be no e-field between the balls (they are at the same potential and you don't get e-fields between objects at the same potential) but there are some on the side facing ground potentials. Unbalanced forces on the balls--->motion of the balls.
First off: Potential is a theoretical construct that you take the gradient of to get the electric field (which is a measurable quantity). It is only indirectly related to charge. You really must understand this; it's extremely important in basic electrostatics. Explanations in terms of field and explanations in terms of potential are interchangeable if you have this understanding.
Also, once you have charged up an object and isolated it, that does NOT mean its potential will be constant. It depends on the surroundings. If the surroundings change, the potential will change.
ALSO, it doesn't matter what you define as reference potential. One thing it does NOT mean is an object with no net charge. Depending on the presence and positioning of other charges, an uncharged object can be at quite a high potential with respect to another uncharged object.
The only exception is if there is a charge inside the inner object (contained ions, say), in which case there will be an opposite charge on the inner object which depresses the potential function, cancelling the potential difference which would otherwise exist between the inner and outer objects due to the presence of the contained charge and the fact that the inner object is closer to it than the outer object.
Now, it is quite correct to say that the protons may be at a different potential than the sphere. In fact, they have to be. See below.
Actually, electrostatics says no such thing - there can easily be fields between like potentials, depending on the geometry. All electrostatics says is that they have to be symmetric, so that a test charge leaving one would arrive at the other with the same energy.
Halfway between your two protons, the field is null (all else being equal). This is a potential trough. At proton A, the field is... well, singular, if the proton is treated as pointlike. But it's a null singularity; there's no preference in any direction, and the field can be treated as zero (meaning the potential resulting solely from proton A is locally flat... you do understand the Principle of Superposition, I hope...?). As a result, the only net effect on proton A is from the field produced by proton B.
One thing you should have learned in freshman electrostatics is that you ignore a charge when calculating the effect produced on it by other charges. As far as proton A is concerned, the only potential gradient (ie: electric field) present is from proton B, and vice versa. They don't see themselves. This is just a simpler version of what I've explained above.
...I think one of the things you've failed to understand is that the protons in your example MUST be at a higher potential than whatever is containing them. It's simple; they're positively charged. This means that in getting closer to them, a positive test charge will gain potential energy. That is, they represent spikes in the potential function.
What IS your background? Just so I know what I'm dealing with...
Dude, are you seriously proposing that modern electrostatics is wrong, and that you have somehow shown this? Energy/potential and fields/forces are not two different theories. They are two completely equivalent ways of looking at the SAME THEORY. Properly understood, they CAN'T contradict one another.
I did this stuff for my M.Sc. I know what I'm talking about.
Yes, the pith balls will now be at a different potential with respect to whatever you're measuring potential with respect to, but this has no physical effect (it's the "+ c" in the integration of the electric field, which is the physical quantity of interest).
Uh... Do you understand what "net" means?chrismb wrote: You keep coming back to the 'net charge' notion. I am talking about an isolated metallic sphere that has a dearth of electrons, such that there is NOT a net charge on it. I do not see why you keep insisting everything must have a net charge? You can remove electrons from an isolated structure and keep all of that within a secondary container that isolates it from fields running to any other potential. Period. It then does not have a zero net charge.
It will then have an undefined 'potential' which might be mathematically defined as the potential difference with an object that does have a net charge.
Also, once you have charged up an object and isolated it, that does NOT mean its potential will be constant. It depends on the surroundings. If the surroundings change, the potential will change.
ALSO, it doesn't matter what you define as reference potential. One thing it does NOT mean is an object with no net charge. Depending on the presence and positioning of other charges, an uncharged object can be at quite a high potential with respect to another uncharged object.
So far so good.OK. So let's set up the mind experiment again. It is reasonable to probe such a mind-system as one can build in intrinsic errors, but I hope you will eventually see there's nothing significant. here we go; you create a spherical capacitor such that a dielectric spherical shell of some thickness has metallic surfaces inside and out. You ground the outer surface and charge the inner to 1.6MV. Also within this device you put the thin walled metallic sphere I was talking about above, and connect this with a wire of negligible thickness, thus negligible capacitance.
No, that's wrong. If you have a metallic object fully contained within another metallic object and CONNECTED TO IT with a wire, then the two objects are at the same potential and the one inside will have no net charge. (Go look up "Faraday cage" - and "net" - before arguing.)The system is held like that for as long as it takes the charge [electrons] on the interior structures to bleed through the dielectric such that at some point the charge on the interior sphere is evenly distributed. Then the thin wire is cut. You should find that the interior thin sphere now lacks some quantity of electrons such that there are more positive charges in it than negative charges, but also the interior lining of the 'capacitor' does not cause the charge that is left in that sphere from becoming non-uniformly distributed.
The only exception is if there is a charge inside the inner object (contained ions, say), in which case there will be an opposite charge on the inner object which depresses the potential function, cancelling the potential difference which would otherwise exist between the inner and outer objects due to the presence of the contained charge and the fact that the inner object is closer to it than the outer object.
Wrong. Gauss' Law. There is no field inside a closed conducting surface, except that provided by charges within it. In other words, the interior potential is flat unless perturbed by a contained charge.I am now going to try to argue with the field and forces argument: Two protons stuck inside this thing now may experience electrostatic forces. Those two protons may have some sort of potential difference with the sphere and thus are repelled by it, both being postively charged.
Now, it is quite correct to say that the protons may be at a different potential than the sphere. In fact, they have to be. See below.
Congratulations. You have just proved that like charges do not repel.So if the protons are at the same potential with respect to the sphere, there would be no electrostatic field between them. Electrostatics says that there is a field between dissimilar potentials, but the protons are at the same potential!! So there will be no electrostatic repulsion.
Actually, electrostatics says no such thing - there can easily be fields between like potentials, depending on the geometry. All electrostatics says is that they have to be symmetric, so that a test charge leaving one would arrive at the other with the same energy.
Halfway between your two protons, the field is null (all else being equal). This is a potential trough. At proton A, the field is... well, singular, if the proton is treated as pointlike. But it's a null singularity; there's no preference in any direction, and the field can be treated as zero (meaning the potential resulting solely from proton A is locally flat... you do understand the Principle of Superposition, I hope...?). As a result, the only net effect on proton A is from the field produced by proton B.
One thing you should have learned in freshman electrostatics is that you ignore a charge when calculating the effect produced on it by other charges. As far as proton A is concerned, the only potential gradient (ie: electric field) present is from proton B, and vice versa. They don't see themselves. This is just a simpler version of what I've explained above.
...I think one of the things you've failed to understand is that the protons in your example MUST be at a higher potential than whatever is containing them. It's simple; they're positively charged. This means that in getting closer to them, a positive test charge will gain potential energy. That is, they represent spikes in the potential function.
Nope. Gauss' Law. See above. The whole interior space is at the same potential (zero field), except for the effect of the protons themselves. You might as well not have bothered with all this capacitor and metal sphere stuff because it doesn't affect anything inside.By the same electrostatic argument, there should be some potential that can be created on the sphere, by removing its electrons in the manner described above, that actually preferentially pushes the two protons together!
Apparently, according to you... I notice that you conveniently left out the part where I said you hadn't fully understood it. You sound like you seized on the caveats without having first grasped the concepts they're supposed to make you cautious about....and freshman electrostatics is wrong?I'm getting the impression that you're operating on a freshman-level electrostatics education
What IS your background? Just so I know what I'm dealing with...
WHAT.The problem is that every time a critique of a part of my argument is made, that critique RELIES on a description of the proposition of fields and forces. If you begin with proposition A, then use that to consider proposition B, you will ALWAYS come back to being able to show proposition A is correct in preference to B!! This is an issue of scientific paradigm and using existing orthodoxy to argue paradigm shifts is fraught with all sorts of self-proving, vacuous arguments.
Dude, are you seriously proposing that modern electrostatics is wrong, and that you have somehow shown this? Energy/potential and fields/forces are not two different theories. They are two completely equivalent ways of looking at the SAME THEORY. Properly understood, they CAN'T contradict one another.
I did this stuff for my M.Sc. I know what I'm talking about.
Now, here's part of the problem. The observed facts ARE the forces and fields. If you then go up one level of abstraction, you can come up with energy and potential. The description is completely consistent, and neither one is "second-order" with respect to the other. But Faraday didn't observe energies and plot potentials. He observed forces and plotted field lines.The fields-and-forces arguments being presented against my proposition of energy and configuration are ones that appear to go on to prove themselves, and if so then they prove nothing. What is odd is that I have not made any contention that fields-and-forces is wrong. I've said that there must be a way to contrive the right answer because these are useful and legitimate second-order interpretations. But I've argued that you don't really have to use these second-order observations and that you can actually stick with just the first-order, observed 'facts' if you so chose. Really, that's all that I have tried to argue for.
Look, I explained what happens to the potential in all of the situations you described. If you don't understand how a Faraday cage works you have no business talking about paradigm shifts in what is essentially a fully-understood branch of physics (non-relativistic electrostatics). This is starting to feel like trying to read one of MTKeshe's posts on nasaspaceflight...So there should be no need to go back to fields-and-forces in this debate, you merely have to critisise the logic of energy-and-configuration. Or you could just say 'you can look at it any way you like, I can't see the benefit'. Either is fine, and I rather thought the debate would end up with 'So what!' but we're talking about a fundamental paradigm here which is not easy to see if you're sitting comfortably, right in the middle of that paradigm!
It depends on HOW you gave it the charge. If you specifically charged up the pith balls (as implied by the word "uniform"), the charge has no way of leaving them because string is insulating. They would then repel. If you simply charge up the metal sphere, nothing will happen inside because all the charge will be on the outer surface of the metal.Aero wrote:So are you saying that if, using a conducting string, I hung two pith balls in the center of a conducting spherical shell, then gave the whole thing a uniform positive static charge, the pith balls would not repel?
Yes, the pith balls will now be at a different potential with respect to whatever you're measuring potential with respect to, but this has no physical effect (it's the "+ c" in the integration of the electric field, which is the physical quantity of interest).
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Art Carlson
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I'm sure you guys will sort out your freshman E&M in time, but I thought I would throw out the fact that in quantum mechanics, potentials (both electric and magnetic) have a significance that goes beyond taking derivatives to find fields. For that matter, if you try to do E&M using only charges without talking about fields, you will have trouble talking about electromagnetic radiation.93143 wrote:First off: Potential is a theoretical construct that you take the gradient of to get the electric field (which is a measurable quantity). It is only indirectly related to charge. You really must understand this; it's extremely important in basic electrostatics. Explanations in terms of field and explanations in terms of potential are interchangeable if you have this understanding.
I agree. The debate has run its course and the positions have been stated for judgements to be made on the merits, or otherwise, of the arguments.
It started where I questioned whether forces and fields are 'facts' or are notional interpretations of something observed, and I did not seek to question fields and forces. This second level interpretation needs to be further evolved into third and fourth interpretations, each of which moves away from the reality of observed objects and their actions and reactions, before we can hope to delve into the levels of sophistication required for higher level physics. I'm not quite sure why it prompted such an empassioned defence of fields and forces when I tried hard not to talk about them nor to critisise them!
Just a couple of points, however. Firstly, questioning the standard ideas of electrostatics is not unreasonable. By way of example, do not forget Ampere and Weber's 'longitudinal forces', a topic which lead them to form, but then reject, the basic essentials of "Maxwell's" equations long before Maxwell. Their observations of longitudinal forces in electrostatics does not match conventional electrostatics so they dismissed the current ideas we have as too simplistic, so there IS room for plenty of discussion.
Secondly, why do people like to keep referring to personal matters? What is wrong with just sticking to the facts put down on the table rather than trying to raise whether someone is adequately composed of suitable faculties or knowledge?
Which then leads to the final point, is this a post-graduate forum of those conventionally educated (it certainly doesn't look like it is) or is it also open to those who would like to question and discuss freshman science?
It started where I questioned whether forces and fields are 'facts' or are notional interpretations of something observed, and I did not seek to question fields and forces. This second level interpretation needs to be further evolved into third and fourth interpretations, each of which moves away from the reality of observed objects and their actions and reactions, before we can hope to delve into the levels of sophistication required for higher level physics. I'm not quite sure why it prompted such an empassioned defence of fields and forces when I tried hard not to talk about them nor to critisise them!
Just a couple of points, however. Firstly, questioning the standard ideas of electrostatics is not unreasonable. By way of example, do not forget Ampere and Weber's 'longitudinal forces', a topic which lead them to form, but then reject, the basic essentials of "Maxwell's" equations long before Maxwell. Their observations of longitudinal forces in electrostatics does not match conventional electrostatics so they dismissed the current ideas we have as too simplistic, so there IS room for plenty of discussion.
Secondly, why do people like to keep referring to personal matters? What is wrong with just sticking to the facts put down on the table rather than trying to raise whether someone is adequately composed of suitable faculties or knowledge?
Which then leads to the final point, is this a post-graduate forum of those conventionally educated (it certainly doesn't look like it is) or is it also open to those who would like to question and discuss freshman science?
Uh, thanks, Art... I'd heard about something like that, but I didn't want to over-complicate things. If you're trying to convince a guy that momentum is conserved in a collision between two billiard balls, you don't bring relativity into it...
Wait, I get it. You thought that:
Besides, it's not the electric field BETWEEN charges that produces the force. It's the electric field AT a particular charge resulting from all other charges that produces the force on that particular charge. I explained this in my last post.
You also thought that:
If you then understand how potential differences are related to energy changes, the identity of the force approach and the energy approach should be obvious as well. The nonsensical conclusion that two protons in a vacuum can exist at a finite separation without repelling one another cannot be derived from either approach.
Oh, and that longitudinal force stuff is electrodynamics, not electrostatics, and it really is "second-order". We're discussing electrostatics, which is EXTREMELY well understood.
I disagree.chrismb wrote:I agree. The debate has run its course and the positions have been stated for judgements to be made on the merits, or otherwise, of the arguments.
No, it started when you displayed apparent ignorance of the conceptual difference between charge and potential. Charge is absolute. Potential is relative, and not directly related to charge at all. I'm not sure how this philosophical stuff got jumbled up in it...It started where I questioned whether forces and fields are 'facts' or are notional interpretations of something observed, and I did not seek to question fields and forces.
Wait, I get it. You thought that:
This is only true in the infinitesimal limit. Do you know calculus?an electric field only exists between two points at a differential potential
Besides, it's not the electric field BETWEEN charges that produces the force. It's the electric field AT a particular charge resulting from all other charges that produces the force on that particular charge. I explained this in my last post.
You also thought that:
This is not true at all. Just because a proton in the middle of nowhere has a potential of 1.6 MV with respect to the vacuum infinitely far from it does NOT mean that you can put that proton inside a sphere at 1.6 MV and still have the proton also be at 1.6 MV. I have explained this repeatedly. The proton's potential will go up with the addition of the sphere. If you understand the relationship between potential and electric field, this should be obvious.All parts will be 'at 1.6MV' (whatever than means!!).
If you then understand how potential differences are related to energy changes, the identity of the force approach and the energy approach should be obvious as well. The nonsensical conclusion that two protons in a vacuum can exist at a finite separation without repelling one another cannot be derived from either approach.
It's open to all. I posted that particular qualification so that you would know you weren't arguing with a pigheaded ignoramus. For example, Art Carlson is known widely around here as a real-life expert in plasma and fusion physics; as a result, his criticism carries a lot more weight than it would if we thought he was a random troll. Not everyone can tell someone is an expert just from his posts.is this a post-graduate forum of those conventionally educated (it certainly doesn't look like it is) or is it also open to those who would like to question and discuss freshman science?
Oh, and that longitudinal force stuff is electrodynamics, not electrostatics, and it really is "second-order". We're discussing electrostatics, which is EXTREMELY well understood.